1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use as a band filter, and more particularly, to a surface acoustic wave filter having a ladder-type circuit configuration including a plurality of one-port surface acoustic wave resonators.
2. Description of the Related Art
An LiTaO.sub.3 substrate is advantageous in that it has a relatively high electromechanical coefficient and the temperature characteristics thereof are relatively stable. Accordingly, the LiTaO.sub.3 substrate is widely used as the piezoelectric substrate of a surface acoustic wave resonator or a surface acoustic wave filter.
For example, Japanese Unexamined Patent Publication No. 6-188673 discloses a surface acoustic wave filter having a ladder type circuit defined by disposing a plurality of surface acoustic wave resonators on the LiTaO.sub.3 substrate. In this prior art, in order to realize desired impedance characteristics, it is suitable to adjust the numbers of the electrode finger pairs and the apertures (cross widths) in the surface acoustic wave resonators.
Further, Japanese Unexamined Patent Publication No. 5-183380 discloses a surface acoustic wave filter having a ladder type circuit configuration defined by a plurality of surface acoustic wave resonators. It is described in this reference that in order to obtain a desired electrode capacitance, the numbers of the electrode finger pairs and the apertures are carefully selected. The electrode capacitance of the surface acoustic wave resonator is determined substantially by the product of the number of the electrode finger pairs, the aperture, and the dielectric constant of a substrate, as described in Japanese Unexamined Patent Publication No. 5-183380. Accordingly, when the LiTaO.sub.3 substrate is used as the piezoelectric substrate, the product of the number of the electrode finger pairs and the aperture is determined, depending on an objective electrode capacitance.
However, when the surface acoustic wave filter having a ladder type circuit configuration was produced by using a plurality of surface acoustic wave resonators, and for this purpose, the products of the numbers of the electrode finger pairs and the apertures were made constant so as to obtain an electrode capacitance having a desired value, as described above, an unsatisfactory flatness in the pass band was produced. FIG. 17 shows the attenuation frequency characteristics of a conventional surface acoustic wave filter. As shown by an arrow A in FIG. 17, there is a deficiency in the left-side shoulder portion in the pass band. That is, the attenuation was insufficiently reduced in the lower frequency side range in the pass band. Accordingly, the flatness of the attenuation frequency characteristics in the pass band was unsatisfactory.